SR proteins are a family of evolutionaly conserved proteins that are related to the regulation of Pre-mRNA natural and alternative splicing. The N-terminal of mammalian SR proteins consist of one or two specific RNA binding domain (RBD). The first RBD(RBD1) has two characteristic and conserved motifs: RNP-1 and RNP-2. RNP-1 and RNP-2 are involved in interacting with DNA. All members of RBD1 have the characteristic sequence ROAEDA. SR family members that have two RBDs have the characteristic sequence SWQDLKD in the second RBD(RBD2). There are no RNP-1 and RNP-2 motifs in the second RBD(RBD2). The C-terminal of SR proteins contains a domain rich in arginine and serine (RS domain). The SR protein family was named after this domain (A. M. Zahler, et al., 1993; T. Gross, et al., 1999).
SR proteins can bind to so-called enhancer sequence and activate the splicing of the weak upstream intron (Reed, et al., 1996; Staknis et al., 1994). RS domain can be phosphorylated by two protein kinase, Clk/Sty and SRPK1. For RS domain to be activated, the phosphorylation is required (Xiao S. H. and Manley 1997).
In mammals, overexpression of SR splicing factor ASF/SF2 will depress the splicing of pre-mRNA, and so does overexpression of the RS domain in variant (Mount S. M. 1997; Gross T et al., 1999).
Factor Srp20 belongs to a highly conserved SR family. It plays multiple roles in the regulation of natural splicing and alternative splicing. During the assembly of the spliceosome, SR is involved in the positioning of the splicing sites. SR proteins can restore splicing ability to cell extracts that lack splicing activity, (e.g. the S-100 cell extracts). Some SR factors have different effect on some alterative splicing sites (Jumaa H et al., 1997; Jumaa H et al., 1999).
Moreover, inactivation of two SR factor, B52/SRp55 in Drosophila and ASF/SF2 in chicken cell line DT40, are lethal and lead to the death of young embryos. In transgenic mouse comprising Cre-loxp expression of the SRp20 gene was repressed, and implanted preembryo in transgenic mouse can not form into blastophere and will die in morula stage, indicating that Srp20 plays in important role in development. Immunofluorescence shows that Srp20 is expressed in oocyte and early stages of embryo genesis (Ring H Z, et al., 1994; Petersen-Mahrt S K, et al., 1999; Jumaa H, et al., 1999).
The human polypeptide gene in the present invention shares 75% protein-level homology with human SRp20 (database accession #Z85986). The inventive protein has an apparent molecular weight ot 52.2 KD and comprises 475aa). It also contains characteristic domains of the SR family, that is, the N-terminal has one or two specific RNA binding domains (RBD). The first RBD(RBD1) have conserved RNP-1 and RNP-2 motifs, involved in interacting with DNA. All members of RBD1 have characteristic sequence RDAEDA. The SR family members that have two RBDs have the characteristic sequence SWQDLKD in the second RBD(RBD2). There is no RNP-1 and RNP-2 motif in the second RBD(RBD2). The C-terminal of RBD2 has an RS domain rich in arginine and serine (RS domain). The new gene of the present invention is found to be one member of SR family and was named SR splicing factor 52. It was found that the human SR splicing factor 52 has similar biology function as that of the SR splicing factor family.
The discovery of human SR splicing factor 52 polypeptide and its gene provides a method for studying the physiology and biochemistry of cell differentiation and proliferation under normal and pathological conditions. It also provides a new diagnosis and treatment method for diseases caused by abnormality of the cell differentiation and proliferation, including cancer.
As discussed, human SR splicing factor 52 plays an essential role in the regulation of important biological functions such as cell division and embryogenesis, and it's believed that numerous proteins are involved in these regulations. So the identification of the human SR splicing factor 52, especially its amino acid sequence, is always desired in this filed. The isolation of this novel human SR splicing factor 52 builds the basis for research of the protein function under normal and clinical conditions, disease diagnosis and drug development. So the isolation of its cDNA is very important.